Friends Who Betray

What i dont understand is why someone has to claim they are you friends and they stab you in the back. I have this one friend, I'll call her Wendy to protect who she really is. Well my boyfriend and i just broke up. And now Wendy all of a sudden wants to date him. One day i was late to cheer practice and Wendy was called out for her belly-button piercing. The next day at school she accused me of telling our coach she had one just because i came in late. What kind of friend is she i mean why would you do that to your friends. I honestly think that people shouldn't talk about people like they do now a days. I just can't believe that someone i knew since first grade would do that to me. It's amazing what people do just to be seen as cool.